This invention relates to vehicles which ride on crawler tracks and more particularly to vehicles of this type which have more than the customary two track assemblies in order to enhance operational performance.
Crawler vehicles exhibit markedly superior performance characteristics in certain specific respects in comparison with wheeled vehicles. Crawler vehicles tend to be heavier and more complex and less adaptable to high-speed travel on a firm, level roadbed, but these factors are offset in many circumstances by the reduced ground pressure per unit area and the greater flotation, traction and stability on uneven or soft terrain which are provided by tracks as opposed to wheels. Consequently, crawler tracks are extensively used on certain forms of tractors, earth-moving equipment and on various other types of off-highway vehicle.
Most crawler vehicles are equipped with a single pair of crawler track assemblies, each one of the pair extending lengthwise of the vehicle body at an opposite side of the vehicle. Resilient suspensions may be provided which allow a very limited degree of independent oscillation of the two track assemblies to accommodate to terrain irregularities and to reduce road shocks. Most commonly both track assemblies are pivoted at the back to the vehicle body or frame to enable the forward portions to rise and fall independently of each other to a very limited extent as determined by suspension elements and stops near the front of the vehicle.
Much of the operational advantages of crawler tracks arise directly or indirectly from the relatively large ground contact area of such mechanisms. Thus it might appear at first consideration that the above-discussed advantages of crawler tracks in general could be enhanced by simply enlarging the conventional two track assemblies to provide increased ground contact area. In practice, a simple enlargement of the conventional two tracks may not produce the desired results except possibly in the unusual case of a vehicle designed to travel only in a straight line on perfectly flat terrain. If the underlying terrain is uneven and if different portions of the undersurface of the tracks are incapable of sizable independent vertical and angular movements, then at any given time the actual area of the tracks which is in contact with the ground may be much less than the total area of the underside of the track assemblies. Under such circumstances the objective of reduced ground pressure per unit area, greater flotation and traction are not realized and may even be adversely affected. A simple enlargement of the conventional two tracks may also aggravate problems, such as slippage, in connection with turning of the vehicle.
Because of the foregoing problems, it has heretofore been proposed to increase the area of crawler track in contact with the ground by employing more than two track assemblies below the vehicle rather than by simply enlarging the conventional two track assemblies. As heretofore designed, these multi-track assembly vehicles have tended to be structurally complex while not fully realizing the advantages of an extended ground contact area because of a limited ability to adjust to irregularities in the underlying terrain. Where the several track assemblies are unable to oscillate a sizable distance in the vertical direction relative to each other and are unable to assume different angular orientations about longitudinal axes through the vehicle, the mechanism still tends to experience the above-discussed problems on irregular terrain. That is, at any given time only limited portions of the total underside area of the track assemblies may be in effective contact with the ground. In addition, the actual contact areas may shift about randomly because of variations in the contour of the terrain.